34th Annual Scientific Meeting proceedings

Arthrodesis of the proximal interphalangeal joint (PIP) is one of the most commonly performed arthrodeses in horses because it provides a solution to problems in this joint that are not (or no longer) medically treatable and the operated horses retain a reasonable chance of being active again as a sport horse after surgery. As a result, the surgical technique is well described and there is a fair amount of scientific literature available on the outcome after surgery and on the differences in firmness of different constructions and between different types of implants.

The PIP joint in horses is generally considered to be a low-motion and high-load joint. However, kinematic studies have shown that the PIP joint in horses at trot has a range of 14 degrees between full extension and flexion.¹ In addition, this joint is part of the complex anatomy of the horse's foot that has to endure extreme forces during athletic performance. We must therefore assume that there is a complex combination of forces acting on the PIP joint with high peaks of compression, flexion and torsion and that permanently fixating this joint will have an impact on the adjacent joints and tendinous structures.

The main indication for an arthrodesis of the PIP joint is osteoarthritis that is unresponsive to medical treatment. This arthritis may be primarily due to the type of activity performed by the horse or secondary to trauma, presence of osteochondral fragments or bone cysts or as a result of an intra-articular wound or infectious arthritis. Additional indications for an arthrodesis are luxation or subluxation and fractures of the first or second phalanx.

In the majority of cases, an open technique with section of the collateral ligaments and removal of the articular cartilage followed by stable fixation is preferred over other techniques. The author uses routinely a 3-hole 4.5 mm locking plate (PIP-LCP) in combination with two or more transarticular screws. The PIP-LCP was designed specifically for PIP joint arthrodesis. The two proximal screw holes are combi-holes that allow both the use of cortical screws in dynamic compression and locking screws. The distal screw hole is is a stacked combi-hole that can be filled with a locking screw or a cortical screw but not in compression. The increased distance between the middle and distal screw hole allows the most solid part of the plate to be placed over the PIP joint. A locking screw can then be placed in the distal hole just below the subchondral bone plate of the second phalanx. Finally, the distal tapered end of the plate is eliminated, reducing the chance that the plate will interfere with the distal interphalangeal (DIP) joint.

The PIP plate provides dorsal stabilization and compression of the joint. To neutralize the tensile forces on the palmar or plantar side of the joint, two or more transarticular 5.5 screws are used. In large Warmblood horses, there is enough space to place more than two transarticular screws. This provides additional compression and faster complete fusion. To have enough space to place these screws, the first two transarticular screws should be placed just next to the plate and the additional screws then slightly more abaxial and angling towards the abaxial margin of the palmar or plantar eminence.

An arthrodesis of the PIP joint can also be performed minimally invasively without cutting the collateral ligaments and opening the joint. The cartilage is then usually removed by drilling a fan-shaped pattern in a lateromedial direction using a 4.5 drill bit. A fixation is then provided either with transarticular screws or a screw-plate combination. This method has several advantages: shorter operation time, less risk of infection, less long immobilization and hospitalization and reduced cost. The main disadvantage of this technique is delayed bony fusion, presumably because the articular cartilage cannot be removed as much. These techniques are therefore best reserved for horses with advanced osteoarthritis. Recently, a minimally invasive technique using transarticular screws on the standing horse was described.²

In case of fractures of the palmar or plantar eminences or in case of a comminuted fracture of P2, arthrodesis is best performed with two plates and additional compression screws.

Post-operatively, a distal limb cast is applied. Given the increased strength of the PIP-LCP screw construction, the general recommendation in the literature is two weeks. In our practice, we leave the cast on slightly longer (4 weeks) in large Warmblood horses if the horse is comfortable and we do not suspect pressure injuries. This way, we hope for less micro-movements in the joint, a faster joint fusion and eventually less periosteal new bone formation.

The success rates found in the literature vary, but are generally quite good. However, differing patient populations (Quarter Horses vs Warmbloods) and criteria used to gauge succes may have an important impact on the outcome. In performance horses, the outcome seems significantly better for arthrodesis of the hindlimb (73%) compared to forelimbs (25%).³ Additionally, Warmbloods suffer regularly from pain in the DIP joint in the early post-operative phase while the construction appears radiographically perfect. For horses moving at walk and trot on a flat surface, the most important loss of motion following PIP arthrodesis is extension in late stance, which is likely to be compensated primarily by the DIP joint.¹ This increase in DIP joint extension may explain the exacerbation of DIP joint pain following PIP arthrodesis. An intra-articular treatment of the DIP joint in the early post-operative phase can improve the comfort during the rest of rehabilitation.

1. Clayton, H.M., Sha, D.H., Stick, J.A., Robinson, P. (2007). 3D kinematics of the interphalangeal joints in the forelimb of walking and trotting horses. Vet. Comp. Orthopedic. Traumatol. 20: 1-7
2. Heaton, K., Farnsworth, K.D., Souza, C.R.S., Jones, A.R.E. (2019). A standing percutaneous technique for proximal interhphalangeal joint arthrodesis in twelve horses (2014-2017). Vet. Comp. Orthopedic. Traumatol. 32: 165-170
3. Herthel, T.D., Rick, MC., Judy, C.E., Cohen N.D., Herthel, D.J. (2016). Retrospective analysis of factors associated with outcome of proximal interphalangeal joint arthrodesis in 82 horses including Warmblood and Thoroughbred sport horses and Quarter Horses (1992-2014). Equine Vet. J. 48(5): 557-64